Here is a trace involving generator expressions.  Using 3.10.0b3 on Mac, there are "line" events within the expression.  Those events are missing on Windows.

--- 8< -------------------------------
import linecache, sys

def trace(frame, event, arg):
    # The weird globals here is to avoid a NameError on shutdown...
    if frame.f_code.co_filename == globals().get("__file__"):
        lineno = frame.f_lineno
        print("{} {}: {}".format(event[:4], lineno, linecache.getline(__file__, lineno).rstrip()))
    return trace

def doit():
    o = ((1,2), (3,4))
    o = (a for a in o)
    for tup in o:
        x = tup[0]
        y = tup[1]

print(sys.version)
sys.settrace(trace)
doit()
---------------------------------------

When run on Mac, it produces this output:


3.10.0b3 (default, Jun 18 2021, 06:43:38) [Clang 12.0.0 (clang-1200.0.32.29)]
call 10: def doit():
line 11:     o = ((1,2), (3,4))
line 12:     o = (a for a in o)
line 13:     for tup in o:
call 12:     o = (a for a in o)
line 12:     o = (a for a in o)
retu 12:     o = (a for a in o)
line 14:         x = tup[0]
line 15:         y = tup[1]
line 13:     for tup in o:
call 12:     o = (a for a in o)
line 12:     o = (a for a in o)
retu 12:     o = (a for a in o)
line 14:         x = tup[0]
line 15:         y = tup[1]
line 13:     for tup in o:
call 12:     o = (a for a in o)
retu 12:     o = (a for a in o)
retu 13:     for tup in o:


When run on Windows, it produces this output:

3.10.0b3 (tags/v3.10.0b3:865714a, Jun 17 2021, 20:39:25) [MSC v.1929 64 bit (AMD64)]
call 10: def doit():
line 11:     o = ((1,2), (3,4))
line 12:     o = (a for a in o)
line 13:     for tup in o:
call 12:     o = (a for a in o)
retu 12:     o = (a for a in o)
line 14:         x = tup[0]
line 15:         y = tup[1]
line 13:     for tup in o:
call 12:     o = (a for a in o)
retu 12:     o = (a for a in o)
line 14:         x = tup[0]
line 15:         y = tup[1]
line 13:     for tup in o:
call 12:     o = (a for a in o)
retu 12:     o = (a for a in o)
retu 13:     for tup in o:

On Windows, the "line 12" events are missing.

Ned, is this a regression (does 3.9 do the right thing on Windows) or an inconsistency between Mac and Windows?

I suspect this might have something to do with the PREDICT macros. If that the were the case 3.9 should show the same inconsistency between Windows and the Mac.

This happens with all the 3.10 betas, and does not happen in 3.9 and earlier:
https://github.com/nedbat/coveragepy/runs/2864611225
(Sorry, other failures with earlier 3.10 betas obscured the mac/win difference.)

I think this is a combination of https://bugs.python.org/issue44297 and the PREDICT macros.

I don't have a  windows machine to confirm this on, but I suspect that if you rewrite `doit` as:

def doit():
    o = ((1,2), (3,4))
    o = (a for
         a in
         o)
    for tup in o:
        x = tup[0]
        y = tup[1]

then you should be able to observe a difference between Windows and Mac on 3.9 as well.

I tried adding that rewritten doit as a new test, and it does not show a       mac/win difference on 3.9. In fact, it doesn't show a mac/win difference on 3.10!

https://github.com/nedbat/coveragepy/actions/runs/956791631

Hmm, I'm a bit puzzled by that.

Did you test with 3.10b3 or the latest build from the 3.10 branch with the fix to https://bugs.python.org/issue44297 included?

This was with 3.10.0b3.  I haven't got a way (yet) to build with my own-built versions of CPython.

Ned, is this still an issue with the release candidate of 3.10?

Looks like this is fixed with 3.10.0rc1, thanks.